1. The Field of the Invention
The present invention relates generally to x-ray tubes that employ a target anode rotatably supported by a bearing assembly. More particularly, embodiments of the present invention relate to systems and structures concerned with improving the rate that heat is transferred away from the x-ray tube bearing assembly and thereby minimize destructive thermal conditions that occur during operation of the x-ray tube.
2. The Relevant Technology
X-ray producing devices are valuable tools that are used in a wide variety of industrial, medical, and other applications. For example, such devices are commonly used in areas such as diagnostic and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis and testing. While they are used in various applications, the different x-ray devices share the same basic underlying operational principles. In general, x-rays, or x-ray radiation, are produced when electrons are emitted, accelerated, and then impinged upon a material of a particular composition.
Typically, these processes are carried out within a vacuum enclosure. Disposed within the vacuum enclosure is an electron source, or cathode, and an anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes a stream of electrons to be emitted by the process of thermionic emission. A high voltage potential applied across the anode and the cathode causes the electrons emitted from the cathode to rapidly accelerate towards a target surface, or focal track, positioned on the anode.
The accelerating electrons in the stream strike the target surface, typically a refractory metal having a high atomic number, at a high velocity and a portion of the kinetic energy of the striking electron stream is converted to electromagnetic waves of very high frequency, or x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as the body of a patient. As is well known, the x-rays can be used for therapeutic treatment, x-ray medical diagnostic examination, material analyses, or other procedures.
In addition to stimulating the production of x-rays, the kinetic energy of the striking electron stream also causes a significant amount of heat to be generated. Some of this heat is often conducted to other areas of the x-ray tube and, as discussed further below, can result in thermal stresses that damage the tube.
In addition to the heat generated as a result of the primary electron stream, other sources of destructive heat are present within the operating x-ray tube. For example, a percentage of the electrons that strike the target surface of the anode do not generate x-rays, and instead simply rebound from the surface and then impact other surfaces and structures within the x-ray tube evacuated enclosure. These are often referred to as xe2x80x9csecondaryxe2x80x9d electrons. These secondary electrons retain a large percentage of their kinetic energy after rebounding, and when they impact non-target surfaces, a significant amount of heat is generated that is conducted to various other elements, such as the bearing assembly, of the x-ray device. Thus, non-target structures, as well as the anode, are routinely exposed to extremely high operating temperatures.
The heat produced by secondary electrons combined with the high temperatures generated at the target anode, often reaches levels high enough to damage portions of the x-ray tube structure and components. In fact, the resulting thermal stresses often shorten the operational life of the x-ray device, affect its efficiency and performance, and/or render it inoperable. These high temperatures can be especially problematic in rotating anode type x-ray tubes.
In a typical rotating anode type x-ray tube, the anode is mounted to a shaft of a bearing assembly confined within a bearing housing. Generally, the bearing assembly includes front and rear bearings having respective sets of balls confined within front and rear races disposed circumferentially with respect to the shaft. Because the balls are free to travel along the races, the shaft of the bearing assembly can freely rotate but is desirably constrained from any substantial axial movement. A stator serves to impart rotational movement to the shaft and the connected anode. As the anode rotates, each point on the focal track is rotated into and out of the path of the electron beam generated by the cathode. In this way, the electron beam is in contact with a focal spot on the focal track for only short periods of time, thereby allowing the remaining portion of the focal track to cool during the time that it takes such given portion to rotate back into the path of the electron beam.
The rotating anode x-ray tube of this sort is used in a variety of applications, some of which require that the anode be rotated at relatively high speeds so as to maintain an acceptable heat distribution along the focal track. For instance, x-ray tubes used in mammography equipment have typically been operated with anode rotation speeds around 3500 revolutions per minute (rpm). However, the demands of the industry have continued to change and high-speed machines for mammography and other applications are now being produced that operate at anode rotation speeds of around 10,000 rpm and higher. Moreover, the rotation must be exact; any wobble or non-uniform rotation of the anode greatly reduces the operating efficiency of the x-ray tube, or may render it imoperable. These high rotational speeds, coupled with the need for rotational precision, make the rotating anode structurexe2x80x94especially the bearing assemblyxe2x80x94especially susceptible to the high operating temperatures.
For example, high operating temperatures can result in undesirable temperature differentials in the bearing assembly. Because the front bearing is located relatively closer to the anode than the rear bearing, the front bearing is exposed to relatively higher temperatures than is the rear bearing. Since the heat transmitted to the bearing assembly from the anode is not evenly distributed and dissipated, such an arrangement results in a temperature differential between the front and rear bearings. The relatively higher temperature experienced at the front bearing effectively reduces the maximum bulk operating temperature of the anode to a point somewhat lower than what the anode could be safely exposed if at least some of the heat experienced at the front bearing was more evenly distributed or otherwise dissipated. This effectively limits the operating power of the x-ray tube.
One solution to this problem is to use a relatively larger anode having a higher heat absorption capability. However, larger anodes are undesirable due to higher costs and because they are heavier and more difficult to balance and rotate at higher speeds.
In addition to acting as a limitation on the maximum operational temperature of the anode, the temperature differential between the front and rear bearings also has negative implications with respect to the operation of the bearings, and thus, the x-ray device as a whole. In particular, because thermal expansion is at least partially a function of temperature, the relatively greater temperature at the front bearing results in a relatively greater expansion of the front bearing, considered with respect to the expansion of the rear bearing. A thermal expansion differential between the front and rear bearings, can cause unbalanced, or otherwise improper, rotation and operation of the shaft which is supported by the bearings. Unbalanced shaft rotation, or similar defects, may cause, among other things, undesirable drifting or movement of the focal spot and degradation of resulting x-ray image quality.
Not only are temperature differentials in the bearings associated with various undesirable and destructive effects, but excessively high temperatures, in general, have a variety of undesirable consequences with respect both to the life and operation of the bearings, and thus of the x-ray device as a whole. For example, extreme operating temperatures may cause increased vibration and noise in the bearing assembly. Such noise and vibration are further exacerbated by the high rotational speeds of the rotating anode. Bearing noise and vibration are undesirable because they can be unsettling to a patient, particularly in applications such as mammography where the patient is in intimate contact with the x-ray machine. Moreover, noise and vibration may be distracting to the x-ray machine operator. Also, unchecked vibration can shorten the operating life of the x-ray tube over time. Finally, the quality of the images produced by the x-ray device is at least partly a function of the stability of the focal spot on the target surface. Thus, vibration may compromise the quality of the x-ray image by causing undesirable movement of the focal
High rotational speeds and high operating temperatures cause vibration and noise in the bearing assembly for a number of reasons. For example, high temperatures can melt the thin film metal lubricant, typically silver or lead, that is present on the bearing surfaces. When the bearings cool, the metal lubricant may clump and create rough spots in the races. Upon subsequent start-up of the x-ray device, the balls travel at high speeds over the rough spots in the races, thereby causing vibration and noise. Moreover, repeated exposure to high temperatures can degrade the bearings, thereby reducing their useful life, as well as that of the x-ray tube.
Heat may be especially problematic depending on the physical arrangement of the components in the bearing assembly and bearing housing, and the materials from which those components are constructed. In particular, in some known designs, operating heat is conducted directly to the bearing assembly by way of solid metal parts that collectively form a heat path between the anode and the bearing assembly. Additional heat is also generated in the bearing assembly as a result of bearing friction, which generally increases as operating speeds increase.
The resulting heat can cause the physical connections or interfaces in the shaft and bearing assembly to loosen and vibrate. Loosening can occur when the components of the bearing assembly are constructed of different metals that have different thermal expansion rates. In such a case, the various parts will each expand and contract at different respective rates when heated and cooled.
For example, the bearing housing is typically constructed of copper, or a copper alloy. The bearings, which are generally constructed of a steel alloy are captured in a cavity defined by the housing. As the copper housing heats up, the diameter of the cavity increases more quickly than the outside diameter of the bearings, thereby creating a gap between the bearing and the cavity wall. The gap thus created allows the bearings to move axially within the housing and thereby generate noise and vibration.
Such problems are of particular concern in the new generation of high-power rotating anode x-ray tubes that have relatively higher operating temperatures than the typical devices. In general, high-powered x-ray devices have operating powers that exceed 20 kilowatts (kw).
Various attempts have been made to minimize the thermal stress, strain, vibration, noise, and other consequences of high operating temperaturesxe2x80x94especially in bearing assemblies. In general, such attempts typically have focused on removing heat from the x-ray device through the use of various types of x-ray tube cooling systems. However, such approaches have not been entirely satisfactory in resolving these problems. For example, in a typical liquid cooling arrangement a volume of a dielectric coolant is contained in a reservoir in which the x-ray tube is disposed. An external cooling unit continuously circulates coolant through the reservoir and removes heat transmitted to the coolant by the x-ray tube. However, this approach does not sufficiently remove heat in high-power x-ray tubes, nor is it directed specifically to the unique cooling requirements of the bearing assembly. That is, while such systems remove heat from the x-ray tube, they may nevertheless be ineffective in removing sufficient heat from localized xe2x80x9chot spotsxe2x80x9d such as the bearing assembly. As a result, the bearing assembly may operate improperly and/or fail prematurely, thereby shortening the useful life of the x-ray device.
Other attempts to control the destructive effects of operational heat on the bearing assembly have focused on providing emissive coatings on or near the anode. As in the case of liquid cooling systems however, such approaches suffer from a variety of shortcomings which serve to impair their effectiveness.
For example, the repeated heating and cooling cycles to which the x-ray device components are typically exposed may cause emissive coatings to flake or spall away from the coated surface. This debris can then contaminate other components within the x-ray tube, and lead to the premature failure of such components. Moreover, there is often a thermal xe2x80x9cmismatchxe2x80x9d between the surface of the coated component and the emissive coating. This thermal expansion rate differential tends to weaken the bond between the two materials over time, which can again lead to flaking and spalling of the emissive coating.
With respect to emissive coatings, another complicating factor relates to the coating process. In particular, the coating process must be monitored carefully and subjected to strict quality control standards in order to reduce the likelihood of spalling and related defects that could result from an improperly applied coating. Such monitoring and quality control, while somewhat effective in some cases, may nevertheless add significantly to the manufacturing complexity and overall cost of the x-ray device.
Another attempt to reduce the heat levels in bearing assembly involves the use of heat shields or similar structures interposed between the bearing assembly and the anode. Typically, the heat shield is attached to the underside of the anode, proximate the bearing assembly. Heat radiated from the target is then deflected, or redirected, by the heat shield so that it does not pass into the bearing assembly. While such heat shields are somewhat effective in reducing the amount of heat radiated to the bearing assembly, they fail to address the problem of heat transfer from the target to the bearing assembly by conduction. Thus, known heat shields are of limited effectiveness because they address only one of the vehicles by which heat is transferred to the bearing assembly.
In view of the foregoing problems, and others, it would be an advancement in the art to provide an improved bearing assembly which includes features directed to providing for a relative increase in the rate at which heat is rejected from the bearing assembly, and which thereby contributes to a relative increase in the operational life of the bearing assembly, and thus the operational life of the x-ray device as a whole.
The present invention has been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or adequately resolved by currently available bearing assemblies. Briefly summarized, embodiments of the present invention provide a bearing assembly that includes various features directed to facilitating a relative increase in the rate at which heat is rejected from the bearing assembly.
Embodiments of the present invention are particularly well suited for use in the context of rotating anode type x-ray tubes. However, it will be appreciated that embodiments of the present invention are suitable for use in any environment where it is desired to efficiently and reliably remove heat from bearing assemblies, and related components, that are exposed to high operating temperatures.
In one embodiment of the present invention, a bearing assembly is provided that includes a shaft defining front and rear inner races, arranged circumferentially about the body of the shaft, and each including a respective bearing surface. The bearing surfaces of the inner races defined by the shaft are blackened, preferably by an oxidation process that produces an Fe3O4 (iron oxide) coating on the bearing surfaces. The shaft further includes one or more extended surfaces, preferably disposed circumferentially about the shaft body. In one embodiment of the invention, the extended surface takes the form of an increased shaft diameter.
The bearing assembly additionally includes front and rear outer race elements disposed about the shaft so as to be aligned with the front and rear inner races defined by the shaft. As in the case of the inner races, the front and rear outer races include respective bearing surfaces that are blackened, preferably by an oxidation process that produces an Fe3O4 (iron oxide) coating on the bearing surfaces. The front and rear outer races cooperate with, respectively, the front and rear inner races to confine front and rear sets of balls. Finally, a spacer longitudinally separates the front and rear outer race elements, and thus serves to position such front and rear outer race elements. Preferably, the spacer includes a plurality of extended surfaces proximate to the extended surfaces of the shaft.
In operation, the balls in the front and rear races permit the shaft to rotate freely. Because the balls are confined in the races however, the shaft is desirably constrained from any substantial axial movement. Heat transmitted to the bearing assembly, whether by conduction and/or radiation, is radiated from the shaft of the bearing assembly by way of the extended surfaces of the shaft. The extended surfaces thus facilitate a relative increase in the rate of heat transmission from the shaft. Further, because the spacer preferably includes extended surfaces proximate the extended surfaces of the shaft, the spacer absorbs heat radiated by the shaft. The spacer then conducts the absorbed heat to the bearing housing in which the bearing assembly is received. This heat is then removed, at least indirectly, from the bearing housing, preferably by way of a liquid cooling system. Thus, the shaft and the spacer cooperate to desirably reduce the temperature differential, or thermal gradient, along the shaft, and also facilitate a relatively higher level of heat transfer from the bearing assembly than would otherwise be possible.
The blackened bearing surfaces, particularly those in the front inner and outer races, likewise contribute to the reduction of the thermal gradient. In particular, the enhanced emissivity provides a relative increase in heat transfer away from the bearing surfaces, and the temperature of the front bearing, and other components of the bearing assembly, is accordingly reduced.
To summarize, the improved thermal characteristics of embodiments of the invention have several advantages. The service life and reliability of the bearing assembly, and component parts, is improved by the increased rate of heat transfer facilitated by the various extended surfaces, and blackened surfaces, of the bearing assembly. Further, the extended surfaces permit a relative reduction in the thermal gradient along the length of the shaft, thereby contributing to improved heat distribution through the shaft, and reducing the operating temperatures in the front bearing. The improved rates of heat transfer permit a corresponding increase in the bulk operating temperature of the anode, permitting the use of relatively smaller anodes. These and other features and advantages contribute to an increase in the life of the bearing assembly, and thus of the x-ray device as a whole.
These, and other, features and advantages of the present claimed invention will become more fully apparent from the following description and appended claim, or may be learned by the practice of the invention as set forth hereinafter.